Chemical warfare agents (CWAs) such as nerve and blister agents are expected to pose continuing and growing dangers for the Warfighter in the future. We investigate a novel chemical detection modality, based on a new platform for colorimetric detection of chemical threats incorporated in hollow fibers, which are miniature in two dimensions and extendable (“extrudable”) in the third dimension (along the fiber length). By exploring fibers, and films that can be scaled to a fiber geometry, we will enable a new fiber-based chemical threat detector that can serve in textiles worn by the Warfighter (e.g., uniform), as well as in non-worn textiles and an outlying fence or perimeter for early detection of a threat cloud near an expeditionary shelter, outpost, encampment, or base.

Lightweight, portable solar blankets, constructed from thin film photovoltaics, are of great interest to
hikers, the military, first responders, and third-world countries lacking infrastructure for transporting
heavy, brittle solar cells. These solar blankets, as large as two square meters in area, come close to
satisfying specifications for commercial and military use, but they still have limited absorption due to
insufficient material efficiency, and therefore are large and too heavy in many cases.
Metasurfaces, consisting of monolayers of periodic and semi-random plasmonic particles patterned in
a scalable manner, are explored to enhance scattering into thin photovoltaic films (currently of
significant commercial and military value), in order to enhance absorption and efficiency of solar
blankets. Without nano-enhancement, absorption is limited by the thickness of the thin photovoltaic
active layer in the long-wavelength region. In this study, lithographically patterned, periodic Al
nanostructure arrays demonstrate experimentally a large absorption enhancement, resulting in a
predicted increase in short-circuit current density of at least 35% and as much as 70% for optimized
arrays atop 200-nm amorphous silicon thin films. Optimized arrays extend thin-film absorption to the
near infrared region. This impressive absorption enhancement and predicted increase in short-circuit
current density may significantly increase the efficiency and reduce the weight of solar blankets,
enabling their use for commercial and military applications.

Two scanning electron beam lithography (SEBL) patterning processes have been developed, one positive and one negative tone. The processes feature nanometer-scale resolution, chemical amplification for faster throughput, long film life under vacuum, and sufficient etch resistance to enable patterning of a variety of materials with a metal-free (CMOS/MEMS compatible) tool set. These resist processes were developed to address two limitations of conventional SEBL resist processes: (1) low areal throughput and (2) limited compatibility with the traditional microfabrication infrastructure.

In recent years, optical super-resolution by microspheres and microfibers emerged as a new paradigm in nanoscale label-free and fluorescence imaging. However, the mechanisms of such imaging are still not completely understood and the resolution values are debated. In this work, the fundamental limits of super-resolution imaging by high-index barium-titanate microspheres and silica microfibers are studied using nanoplasmonic arrays made from Au and Al. A rigorous resolution analysis is developed based on the object’s convolution with the point-spread function that has width well below the conventional (~&lambda;/2) diffraction limit, where &lambda; is the illumination wavelength. A resolution of ~&lambda;/6-&lambda;/7 is demonstrated for imaging nanoplasmonic arrays by microspheres. Similar resolution was demonstrated for microfibers in the direction perpendicular to the fiber axis with hundreds of times larger field-of-view in comparison to microspheres. Using numerical solution of Maxwell’s equations, it is shown that extraordinary close point objects can be resolved in the far field, if they oscillate out of phase. Possible super-resolution using resonant excitation of whispering gallery modes is also studied.

Unlike a semiconductor, where the absorption is limited by the band gap, a “microrectenna array” could theoretically very efficiently rectify any desired portion of the infrared frequency spectrum (25 - 400 THz). We investigated vertical metal-insulator-metal (MIM) diodes that rectify vertical high-frequency fields produced by a metamaterial planar stripe-teeth Al or Au array (above the diodes), similar to stripe arrays that have demonstrated near-perfect absorption in the infrared due to critical coupling [1]. Using our design rules that maximize asymmetry (and therefore the component of the electric field pointed into the substrate, analogous to Second Harmonic Generation), we designed, fabricated, and analyzed these metamaterial-based microrectenna arrays. NbOx and Al2O3 were produced by anodization and ALD, respectively. Smaller visible-light Pt-NbOx-Nb rectennas have produced output power when illuminated by visible (514 nm) light [2].
The resonances of these new Au/NbOx/Nb and Al/Al2O3/Al microrectenna arrays, with larger dimensions and more complex nanostructures than in Ref. 1, were characterized by microscopic FTIR microscopy and agreed well with FDTD models, once the experimental refractive index values were entered into the model. Current-voltage measurements were carried out, showed that the Al/Al2O3/Al diodes have very large barrier heights and breakdown voltages, and were compared to our model of the MIM diode. We calculate expected THz-rectification using classical [3] and quantum [4] rectification models, and compare to measurements of direct current output, under infrared illumination.
[1] C. Wu, et. al., Phys. Rev. B 84 (2011) 075102.
[2] R. M. Osgood III, et. al., Proc. SPIE 8096, 809610 (2011).
[3] A. Sanchez, et. al., J. Appl. Phys. 49 (1978) 5270.
[4] J. R. Tucker and M. J. Feldman, Rev. of Mod. Phys. 57, (1985)1055.

Nanoparticles and nanostructures with plasmonic resonances are currently being employed to enhance the efficiency of solar cells. <sup>1-3</sup> Ag stripe arrays have been shown theoretically to enhance the short-circuit current of thin silicon layers. <sup>4</sup> Monolayers of Ag nanoparticles with diameter d &lt; 300 nm have shown strong plasmonic resonances when coated in thin polymer layers with thicknesses &lt; d.<sup>5</sup> We study experimentally the diffuse vs. specular scattering from monolayer arrays of Ag nanoparticles (spheres and prisms with diameters in the range 50 – 300 nm) coated onto the front side of thin (100 nm &lt; t &lt; 500 nm) silicon films deposited on glass and flexible polymer substrates, the latter originating in a roll-to-roll manufacturing process. Ag nanoparticles are held in place and aggregation is prevented with a polymer overcoat. We observe interesting wavelength shifts between maxima in specular and diffuse scattering that depend on particle size and shape, indicating that the nanoparticles substantially modify the scattering into the thin silicon film.

Tunable, narrow-wavelength spectral filters with a ms response in the mid-wave/long-wave infrared (MW/LWIR) are an enabling technology for hyperspectral imaging systems. Few commercial off-the-shelf (COTS) components for this application exist, including filter wheels, movable gratings, and Fabry-Perot (FP) etalon-based devices. These devices can be bulky, fragile and often do not have the required response speed. Here, we present a fundamentally different approach for tunable reflective IR filters, based on coupling subwavelength plasmonic antenna arrays with liquid crystals (LCs). Our device operates in reflective mode and derives its narrow bandwidth from diffractive coupling of individual antenna elements. The wavelength tunability of the device arises from electrically-induced re-orientation of the LC material in intimate contact with antenna array. This re-orientation, in turn, induces a change in the local dielectric environment of the antenna array, leading to a wavelength shift. We will first present results of full-field optimization of micron-size antenna geometries to account for complex 3D LC anisotropy. We have fabricated these antenna arrays on IR-transparent CaF2 substrates utilizing electron beam lithography, and have demonstrated tunability using 5CB, a commercially available LC. However, the design can be extended to high-birefringence liquid crystals for an increased tuning range. Our initial results demonstrate &lt;60% peak reflectance in the 4- 6 μm wavelength range with a tunability of 0.2 μm with re-orientation of the surface alignment layers. Preliminary electrical switching has been demonstrated and is being optimized.

Nanoparticles and nanostructures with plasmonic resonances are currently being employed to
enhance the efficiency of solar cells. Ag stripe arrays have been shown theoretically to enhance the
short-circuit current of thin silicon layers. Such Ag stripes are combined with 200 nm long and 60
nm wide “teeth”, which act as nanoantennas, and form vertical rectifying metal-insulator-metal
(MIM) nanostructures on metallic substrates coated with thin oxides, such as Nb/NbO<sub>x</sub> films. We
characterize experimentally and theoretically the visible and near-infrared spectra of these “stripeteeth”
arrays, which act as microantenna arrays for energy harvesting and detection, on silicon
substrates. Modeling the stripe-teeth arrays predicts a substantial net a.c. voltage across the MIM
diode, even when the stripe-teeth microrectenna arrays are illuminated at normal incidence.

As next generation immersion lithography, combined with double patterning, continues to shrink feature sizes, the
industry is contemplating a move to non-chemically amplified resists to reduce line edge roughness. Since these resists
inherently have lower sensitivities, the transition would require an increase in laser exposure doses, and thus, an increase
in incident laser fluence to keep the high system throughput.
Over the past several months, we have undertaken a study at MIT Lincoln Laboratory to characterize performance
of bulk materials (SiO<sub>2</sub> and CaF<sub>2</sub>) and thin film coatings from major lithographic material suppliers under continuous
193-nm laser irradiation at elevated fluences. The exposures are performed in a nitrogen-purged chamber where samples
are irradiated at 4000 Hz at fluences between 30 and 50 mJ/cm<sup>2</sup>/pulse. For both coatings and bulk materials, in-situ laser
transmission combined with in-situ laser-induced fluorescence is used to characterize material performance. Potential
color center formation is monitored by ex-situ spectrophotometry. For bulk materials, we additionally measure spatial
birefringence maps before and after irradiation. For thin film coatings, spectroscopic ellipsometry is used to obtain
spatial maps of the irradiated surfaces to elucidate the structural changes in the coating.
Results obtained in this study can be used to identify potential areas of concern in the lens material performance if
the incident fluence is raised for the introduction of non-chemically amplified resists. The results can also help to
improve illuminator performance where such high fluences already occur.

MIT Lincoln Laboratory has developed a concept that could enable remote (10s of meters) detection of trace
explosives' residues via a field-portable laser system. The technique relies upon laser-induced photodissociation of
nitro-bearing explosives into vibrationally excited nitric oxide (NO) fragments. Subsequent optical probing of the first
vibrationally excited state at 236 nm yields narrowband fluorescence at the shorter wavelength of 226 nm. With proper
optical filtering, these photons provide a highly sensitive explosives signature that is not susceptible to interference from
traditional optical clutter sources (e.g., red-shifted fluorescence). Quantitative measurements of trace residues of TNT
have been performed demonstrating this technique using a breadboard system, which relies upon a pulsed optical
parametric oscillator (OPO) based laser. Based on these results, performance projections for a fieldable system are made.

The need to extend 193nm immersion lithography necessitates the development of a third generation (Gen-3) of
high refractive index (RI) fluids that will enable approximately 1.7 numerical aperture (NA) imaging. A multi-pronged
approach was taken to develop these materials. One approach investigated the highest-index organic thus far
discovered. The second approach used a very high refractive index nanoparticle to make a nanocomposite fluid.
This report will describe the chemistry of the best Gen-3 fluid candidates and the systematic approach to their
identification and synthesis. Images obtained with the Gen-3 fluid candidates will also be presented for a NA &ge; 1.7.

A potential extension of water-based 193-nm immersion lithography involves transition to a higher refractive index
organic immersion fluid coupled with a higher index last lens element. While considerable progress has been made in
improving the photo-durability of the immersion fluid itself, photo-induced contamination of the last lens element
caused by laser exposure in the presence of such organic fluids remains a major concern. In this work, we study
remediation strategies for such contamination, which would be compatible with conventional lithographic production
environments. In general, surface photocontamination layers were found to be highly graphitic in nature, where the
first monolayer is strongly bound to the substrate. We have attempted to develop a surface passivation treatment for
altering the monolayer chemistry and preventing large-scale contamination, but found such treatments to be unstable
under laser irradiation. On the other hand, using hydrogen peroxide as a in-situ cleaning solution has been shown to be
extremely effective. We also present first laser-based durability results of LuAG, which is a leading candidate
material for high index last element to be used with high index fluids.

An extension of water-based immersion lithography involves replacing water with a higher index transparent oil.
Understandably, potential lens contamination is a major concern for an <i>all-organic</i> immersion fluid. We have
constructed an experimental system for controlled irradiation of high index fluids, including capabilities for in-situ
cleaning of potential deposits. We present results of laser-irradiation of several high index immersion fluid candidates.
Using properly developed exposure metrics, we discuss implications for fluid lifetimes in an immersion system, with and
without in-situ purification. Using our in-situ metrology, we are able to decouple bulk fluid degradation from window
photocontamination for several fluids. We find a significant variation in optics contamination rate depending on the fluid
tested. Even the slowest observed contamination rates would require some remediation strategies to remove the built-up
deposit from the final element surface. We also present results of irradiation of model hydrocarbon compound fluids.
Irradiation of these materials leads to fundamental understanding of underlying photochemistry, and also provides
guidance in designing future generation high index fluids.

Combining optical and electron beam exposures on the same wafer level is an attractive approach for extending
the usefulness of current generation optical tools. This technique requires high-performance hybrid resists that perform
equally well with optical and e-beam tools. In this paper Rohm and Haas EPIC<sup>TM</sup> 2340, a 193-nm chemically amplified
photoresist, is used in a hybrid exposure role. The e-beam tool was used to pattern 45 nm half-pitch features and a 193-
nm immersion stepper was used to pattern 60-nm half-pitch features in the same resist layer. The effects of processing
parameters and delay times were investigated.

Leaching of resist components into water has been reported in several studies. Even low dissolution levels of photoacid generator (PAG) may lead to photocontamination of the last optical surface of the projection lens. To determine the impact of this phenomenon on optics lifetime, we initiate a set of controlled studies, where predetermined amounts of PAG are introduced into pure water and the results monitored quantitatively. The study identifies the complex, nonlinear paths leading to photocontamination of the optics. We also discover that spatial contamination patterns of the optics are strongly dependent on the flow geometry. Both bare SiO2 surfaces as well as coated CaF2 optics are studied. We find that for all surfaces, at concentrations typical of leached PAG, below 500 ppb, the in situ self-cleaning processes prevent contamination of the optics.

Leaching of resist components into the water has been reported in several studies. Potential effects of photo-acid generator (PAG) dissolved in water include photocontamination of the last optical surface and the formation of particulate defects on the wafer surface. In order to determine the impact of these phenomena on lithographic performance, such as optics lifetime and yield, we have initiated a set of controlled studies, where predetermined amounts of PAG were introduced into pure water and the results monitored quantitatively. One set of studies identified the complex, nonlinear paths leading to photocontamination of the optics. At concentrations typical of leached PAG, below 500 ppb, the in-situ self-cleaning processes prevent contamination of the optics. On the other hand, initial experiments with a nano-dropper show that micron-scale particles from the dissolved PAG are formed on the wafer surface when water evaporates. This phenomenon requires further systematic studies both at the fundamental science and the engineering levels.

We analyze the performance and process latitudes of a high-throughput, all-optical lithography method that addresses the requirements of the 32-nm node. This hybrid scheme involves a double exposure and only a single photomask. The first exposure forms dense gratings using maskless immersion interference lithography. These regular grating patterns are then trimmed in a second exposure with conventional projection lithography. While the highest resolution features are formed with interference imaging, the trimming operation requires significantly lower resolution. We have performed lithography simulations examining a number of representative 32-nm node patterns; both one-dimensional and two-dimensional. The results indicate that 32-nm node lithography requirements can be met using a hybrid optical maskless (HOMA) approach. Trim photomasks can be two to three generations behind the fine features, while the trim projection tools can be one to two generations behind the fine features. This hybrid optical maskless method has many of the benefits of maskless lithography without the severe throughput challenge of currently proposed maskless technologies.

Pellicle materials for use at 157 nm must display sufficient transparency at this wavelength and adequate lifetimes to be useful. We blended a leading candidate fluoropolymer with silica nanoparticles to examine the effect on both the transparency and lifetime of the pellicle. It is anticipated that these composite materials may increase the lifetime by perhaps quenching reactive species and/or by dilution, without severely decreasing the 157-nm transmission. Particles surface-modified with fluorinated moieties are also investigated. The additives are introduced as stable nanoparticle dispersions to casting solutions of the fluoropolymers. The properties of these solutions, films, and the radiation-induced darkening rates are reported. The latter are reduced in proportion to the dilution of the polymer, but there is no evidence that the nanoparticles act as radical scavengers.

Immersion lithography is proposed as a method for improving optical microlithography resolution to 45 nm and below via the insertion of a high-refractive-index liquid between the final lens surface and the wafer. Because the liquid acts as a lens component during the imaging process, it must maintain a high, uniform optical quality. One potential source of optical degradation involves changes in the liquid's index of refraction caused by changing temperatures during the exposure process. Two-dimensional computational fluid dynamics models from previous studies investigated the thermal and fluid effects of the exposure process on the liquid temperature associated with a single die exposure. We include the global heating of the wafer from multiple die exposures to better represent the "worst-case" liquid heating that occurs as an entire wafer is processed. The temperature distributions predicted by these simulations are used as the basis for rigorous optical models to predict effects on imaging. We present the results for the fluid flow, thermal distribution, and imaging simulations. Both aligned and opposing flow directions are investigated for a range of inlet pressures that are consistent with either passive systems or active systems using filling jets.

The photo-induced degradation of 157-nm anti-reflective (AR) coatings, and the role of water vapor in the ambient, have been studied with in-situ spectroscopic ellipsometry, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy. Using ellipsometric techniques, we find that MgF<sub>2</sub> thin films develop a surface roughness layer under laser irradiation at an incident dose of ~0.1 MJ/cm<sup>2</sup>. These thin film changes occur well before any changes in 157-nm transmission are observed. The findings are confirmed by ex-situ post-irradiation AFM measurements. LaF<sub>3</sub> does not exhibit this effect. Addition of ppm-levels of moisture suppresses surface roughness formation, suggesting that the surface roughness growth may be a precursor to the transmission degradation of full AR stacks that had been observed earlier.

We have measured the intrinsic scattering of water with an eye toward its potential impact on immersion lithography. Quantitative measurements of the elastic Rayleigh scatter agree well with theory and show a loss of 0.001 cm<sup>-1</sup>. Qualitative measurements of the inelastic Raman scattering show a strong peak at 206 nm, consistent with the O-H stretch present in water. Both are expected to contribute flare of < 10<sup>-6 </sup>of the incident intensity. We have also examined the possibility for bubbles in the immersion liquid, and in particular those which form near the resist surface. We have measured scattering from single bubbles and estimate that bubbles as small as 5 &mu;m should be detectable in this fashion. In addition, we have measured the potential for bubbles due to laser induced resist outgassing by direct imaging. In 2500 resist images (~235 mm<sup>2</sup> of surface), we have seen only one bubble candidate which, due to its persistence in the water, we do not believe represents a true outgassing-induced bubble. Finally, using a technique borrowed from biology, rapid cryofixation/freeze fracture, we have examined nanobubbles which form spontaneously on hydrophobic surfaces and found that degassing the water prevents their formation.

The final projection lens element in a 193-nm immersion-based lithographic tool will be in direct contact with water during irradiation. Thus, any lifetime considerations for the lens must include durability data of lens materials and thin films in a water ambient. We have previously shown that uncoated CaF<sub>2</sub> is attacked by water in a matter of hours, as manifested by a substantial increase in AFM-measured surface roughness.<sup>1</sup> Thus, CaF<sub>2</sub> lenses must be protected, possibly by a thin film, and the coatings tested for laser durability in water. To address the above lifetime concerns, we have constructed a marathon laser-irradiation system for testing thin film exposure to water under long-term laser irradiation. Coated substrates are loaded into a custom water cell, made of stainless steel and Teflon parts. Ultrapure water is delivered from a water treatment testbed that includes particle filtration, deionization and degassing stages. In-situ metrology includes 193-nm laser ratiometry, UV spectrophotometry and spectroscopic ellipsometry, all with spatial profiling capabilities. In-situ results are coupled with off-line microscopy, AFM measurements and spatial surface mapping with spectroscopic ellipsometry at multiple incidence angles. A variety of laser-induced changes have been observed, from complete adhesion loss of protective coatings to more subtle changes, such as laser-induced index changes of the thin films or surface roughening. Implications of the study on expected lifetimes of the protective coatings in the system will be discussed.

We have designed and constructed a microstepper for 157 nm immersion lithography. The lens, designed and fabricated at Newport, provides a numerical aperture of 1.3 and a field size of 60 &mu;m with immersion liquids of index n=1.38. Because of a lack of system interferometer, final alignment has been ongoing in the field using actuators incorporated into the lens design. Lithography down to 250 nm has been demonstrated but lens alignment has proved difficult. We are currently implementing an image monitoring system to provide real-time feedback on lens performance and to allow expedited alignment.

In liquid immersion lithography the last optical element is in intimate contact with the liquid for extended periods of time, and therefore is at risk of being contaminated by impurities in the liquid. The purity of the liquid must be kept under stringent control compared to "dry" lithography, since the density of liquid is ~ 1000 times higher than that of gas. Thus, 1 part per billion contaminant in the liquid may have an equivalent effect on the optics to 1 part per million in gas. The risk is that the combination of high contaminant density, short wavelength, and large laser dose will conspire to contaminate the optics, change its transmission, and possibly cause increased flare. In order to clarify the potential for such effects, we have begun a set of experiments with controlled contamination. In these studies, a 193-nm laser irradiates a sample in the presence of flowing clean water into which controlled amounts of contaminant have been injected. The sample is either bare fused silica or calcium fluoride protected with thin films. Results will be presented with organic contaminants such as isopropanol and acetone. These results will include an analysis on the implications for controlling water purity.

Polarization dependent diffraction efficiencies in transmission through gratings on specially designed masks with pitch comparable to the wavelength were measured using an angle-resolved scatterometry apparatus with a 193 nm excimer source. Four masks - two binary, one alternating and one attenuated phase shift mask - were included in the experimental measurements. The validity of models used in present commercially available simulation packages and additional polarization effects were evaluated against the experimental scattering efficiencies.

A fabrication process has been developed which prevents solvent intermixing between layers of diazonaphthoquinone/novolac (DNQ/novolac) based resist. The process enables three-dimensional structures to be batch fabricated stereolithographically using integrated circuit-compatible resist, coating, and exposure techniques, followed by a single development step. To prevent solvent intermixing, a combination of solvent tailoring and surface treatment is employed. The photoresist is first constituted into a weaker, less polar solvent. Before coating a new layer, the surface is exposed to ozone, thus increasing the hydrophilicity of the surface and providing a less soluble barrier layer. This enables the formation of a stack of successively photoimaged layers of the same material, which are then developed in a single step. A new interlayer dose modulation technique to optimize the development process in positive tone resists such as DNQ/novolac is also described.

A simulation package has been developed for predicting the influence of immersion, i.e., the presence of a uniform liquid layer between the last objective lens and the photoresist, on optical projection lithography. This technology has engendered considerable interest in the microlithography community during the past year, as it enables the real part of the index of refraction in the image space, and thus the numerical aperture of the projection system, to be greater than unity. The simulation program described here involves a Maxwell vector solution approach, including polarization effects and arbitrary thin film multilayers. We examine here the improvement in process window afforded by immersion under a variety of conditions, including λ = 193 nm and 157 nm, annular illumination, and the use of alternating phase shift mask technology. Immersion allows printing of dense lines and spaces as small as 45 nm with acceptable process window. We also examine the effect of variations in liquid index on the process window and conclude that the index of the liquid must be known to and maintained within a few parts per million. This has important implications for the temperature control required in future liquid immersion projection systems.

The premise behind immersion lithography is to improve resolution by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer by inserting a high-index liquid in place of the low-index air that currently fills the gap. We present a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer. The objectives of this feasibility study are to identify liquid candidates that meet the fluid mechanical requirements and to verify modeling tools for immersion lithography. The filling process was analyzed to simplify the problem and identify important fluid properties and system parameters. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer are developed and used to investigate a passive technique for filling this gap, in which a liquid is dispensed onto the wafer as a puddle, and then the wafer and liquid move under the lens. Numerical simulations include a parametric study of the key dimensionless groups influencing the filling process, and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements.

Successful insertion of 157-nm lithography into production requires that materials comprising the optical train meet the lifetime requirements of the industry. At present, no degradation of bulk fluoride materials has been observed for at least up to 10<sup>9</sup> pulses. However, last year we reported on the surface damage to fluoride materials that appeared after 3-4x10<sup>9</sup> pulses at moderate fluences of 3-4 mJ/cm<sup>2</sup>/pulse<sup>2</sup>. This damage manifested itself as a precipitous transmission drop of up to 50% at 157 nm and was accompanied by the formation of a porous rough surface layer about 0.20 &#956;m thick. Understanding this surface damage is important for the durability of laser windows and beam delivery optics, and it may also help elucidate fundamental 157-nm photophysics of fluoride surfaces. To understand the underlying phenomena, we have designed and constructed a new accelerated damage test chamber. The chamber utilizes 157-nm light from a lithography-grade laser operating at 1000 Hz. Inside the chamber, light is focused onto the sample to a submillimeter spot size. The chamber allows us to test in-situ transmission of multiple spots on a given sample over a range of fluences up to 140 mJ/cm<sup>2</sup>/pulse without breaking purge. We have used this chamber to understand the scaling of the damage mechanism for both uncoated and antireflectance (AR) -coated CaF<sub>2</sub> samples as a function of laser repetition rate and fluence. Substrate damage appears to be governed by a complex set of mechanisms, both thermal and non-thermal in origin. Preliminary damage studies of AR-coated substrates show that AR-coating related degradation occurs well before the onset of the substrate surface damage.

We present results of the durability of antireflectance (AR) coatings under laser irradiation with emphasis on the interplay between coating materials and ambient. We find that introducing ppm-levels of water has a dramatic impact on the performance of certain coatings. In particular, no significant degradation of a coating was observed for up to 1MJ/cm<sup>2</sup> dose in the presence of ~20 ppm H<sub>2</sub>O, whereas linear transmission drop of several percent was observed when irradiating a coating of similar design in <0.1 ppm H<sub>2</sub>O but under 1.5 ppm O<sub>2</sub>. Cycling water concentration on and off leads a corresponding cycling of transmission of the coatings. Adding water vapor to the ambient has a much greater benefit to coating durability than adding corresponding amounts of gas phase oxygen. In a series of experiments involving the same coating stack with different degrees of porosity of the outer layer, moisture was found to have the greatest impact on the most porous coating.

A simulation package has been developed for predicting the influence of immersion, i.e. the presence of a uniform liquid layer between the last objective lens and the photoresist, on optical projection lithography. This technology has engendered considerable interest in the microlithography community during the past year, as it enables the
real part of the index of refraction in the image space, and thus the numerical aperture of the projection system, to be greater than unity. The simulation program described here involves a Maxwell vector solution approach, including polarization effects and arbitrary thin film multilayers. We examine here the improvement in process window afforded by immersion under a variety of conditions, including &lambda; = 193 nm and 157 nm, annular illumination, and the use of alternating phase shift mask technology. Immersion allows printing of dense lines and spaces as small as 45 nm with acceptable process window. We also examine the effect of variations in liquid
index on the process window and conclude that the index of the liquid must be known to and maintained within a few parts-per-million. This has important implications for the temperature control required in future liquid immersion projection systems.

Photo-induced contamination rates on 157-nm optical surfaces have been studied in controlled experiments with contaminants containing fluorocarbon, sulfur and iodine. The compounds investigated represent species generated in controlled outgassing studies of common construction materials and photoresists used in 157 nm steppers. No photocontamination was measured for highly fluorinated alkanes and ethers on an anti-reflective coating, at levels exceeding 10 ppm. Photocontamination with sulfur based compounds was similar to the behavior observed with hydrocarbon based derivatives. Sulfur containing residues, even from oxidized precursors, are fully cleanable in oxygen, with cleaning rates scaling proportionally with the level of oxygen. In contrast, at elevated levels of oxygen, non-volatile iodate complexes can form from iodine based contaminants. Sulfonium salts should therefore be considered over iodonium species in photoacid generators in 157 nm photoresists. In addition to studying these new classes of compounds, cleaning rates of hydrocarbon residues in trace levels of water were also studied.

An angle-resolved scattering detection system has been designed and implemented for use at 157 nm. This tool will enable the optimization of polishing and thin-film deposition, whith an eye towards minimizing small-angle scatter in projection lithography tools. In this test-bed, scattered rays can be collected to 4° from the directional ray of the specularly transmitted beam (corresponding to spatial wavelengths of surface roughness below 2 &#956;m) over a dynamic range of 7 orders of magnitude, and to 0.5° with a dynamic range of 5 orders of magnitude. The angular scattering distributions in CaF<sub>2</sub> samples and antireflective coatings are compared. From these results, the impact of scattering on image performance in exposure tools at 157 nm is estimated.

The requirements of liquids for use in immersion lithography are discussed. We present simple calculations of the transmission and index homogeneity requirements of the immersion liquid (<i>T</i> > 0.95 and &delta;<i>n</i> < 5&times;10<sup>-7 </sup>respectively for sin &theta; = <i>NA</i>/<i>n</i> = 0.9 and a working distance of 1 mm) along with the temperature and pressure control requirements which follow from them. Water is the leading candidate immersion liquid for use at 193 nm, and we present data on its chemical compatibility with existing 193 nm resists through dissolution/swelling and surface energy studies. We find that it has a minimal impact on at least some current 193 nm resists. At 157 nm, suitably transparent immersion fluids remain to be identified. Perfluorinated polyethers (PFPE) are among the most transparent organics measured. The lowest PFPE absorbance at 157 nm can be further reduced by roughly a factor of two, from 6 to 3 cm<sup>-1 </sup>through removal of dis-solved oxygen. We also discuss our efforts to understand the origin of the remaining absorbance through supercritical CO<sub>2</sub> fractionation.

The premise behind immersion lithography is to improve the resolution for optical lithography technology by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer. This is accomplished through the insertion of a high index liquid in place of the low index air that currently fills the gap. The fluid management system must reliably fill the lens-wafer gap with liquid, maintain the fill under the lens throughout the entire wafer exposure process, and ensure that no bubbles are entrained during filling or scanning. This paper presents a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer in immersion lithography. The objective of this feasibility study was to identify liquid candidates that meet both optical and specific fluid mechanical requirements. The mechanics of the filling process was analyzed to simplify the problem and identify those fluid properties and system parameters that affect the process. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer were developed for simulating the process. The CFD simulations were used to investigate two methods of liquid deposition. In the first, a liquid is dispensed onto the wafer as a “puddle” and then the wafer and liquid move under the lens. This is referred to as passive filling. The second method involves the use of liquid jets in close proximity to the edge of the lens and is referred to as active filling. Numerical simulations of passive filling included a parametric study of the key dimensionless group influencing the filling process and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements. For active filling, preliminary simulation results characterized the influence of the jets on fluid flow.

We present the methodology and recent results on the long-term evaluation of optical materials for 157-nm lithographic applications. We review the unique metrology capabilities that have been developed for accurately assessing optical properties of samples both online and offline, utilizing VUV spectrophotometry with in situ lamp-based cleaning. We describe ultraclean marathon testing chambers that have been designed to decouple effects of intrinsic material degradation from extrinsic ambient effects. We review our experience with lithography-grade 157-nm lasers and detector durability. We review the current status of bulk materials for lenses, such as CaF2 and BaF2, and durability results of antireflectance coatings. Finally, we discuss the current state of laser durability of organic pellicles.

We present the results of a preliminary feasibility study of liquid immersion lithography at 157 nm. A key enabler has been the identification of a class of commercially available liquids, perfluoropolyethers, with low 157 nm absorbance α157 ∼ 10 cm−1 base 10. With 157 nm index of refraction around 1.36, these liquids could enable lithography at numerical aperture ∼1.25 and thus resolution of 50 nm for k1 = 0.4. We have also performed preliminary studies on the optical, chemical, and physical suitability of these liquids for use in high throughput lithography. We also note that at longer wavelengths, there is a wider selection of transparent immersion liquids. At 193 nm, the most transparent liquid measured, de-ionized water, has α193 = 0.036 cm−1 base 10. Water immersion lithography at 193 nm would enable resolution of 60 nm with k1 = 0.4.

An attenuating phase shifting mask has been designed, fabricated, and tested at 157 nm. It consists of two layers, a metal attenuator and a transparent phase shifter. The metal, platinum, was chosen for its chemical and radiation stability. The phase shifter was a commercial spin-on glass. A single step of pattern transfer has been implemented, which significantly simplifies the fabrication process of the mask. The lithographic advantage in increased depth of focus was demonstrated for 130-nm spaces and contacts, and it was found to agree with numerical simulations.

Long-term durability tests of optical thin films and thin films designed for attenuating phase shifters have been performed in a chamber, which stresses clean protocols to eliminate extraneous effects of surface contamination. Most anti-reflective coatings tend to degrade several percent in transmission within 1 MJ/cm2 total dose. Attenuating phase
shifting materials usually show an increase in transmission during 6 kJ/cm2. In both types of films there are exceptions, indicating that there are no fundamental causes that would limit the performance of such films. A new phenomenon of laser-induced surface damage in calcium fluoride has been observed, and is being studied.

The introduction of 157 nm as the next optical lithography wavelength has created a need for new soft (polymeric)
or hard (quartz) pellicle materials. Pellicles should be > 98% transparent to incident 157 nm light and, ideally, sufficiently
resistant to photochemical damage to remain useful for an exposure lifetime of 7.5 kJ/cm2.
The transparency specification has been met. We have developed families of experimental Teflon&trade;AF (TAFx)
polymers with > 98% transparency which can be spin coated and lifted as micron-scale, unsupported membranes. Still higher
transparencies should be possible once optimization of intrinsic (composition, end groups, impurities, molecular weight) and
extrinsic (oxygen, absorbed hydrocarbons, contaminants) factors are completed. The measured transparencies of actual
pellicle films, however, are affected by many factors other than absorption. Film thickness must be precisely controlled so as
to allow operation at the fringe maxima for the lithographic wavelength. Roughness and thickness uniformity are also
critical. An important part of our program has thus been learning how to spin membranes from the solvents that dissolve our
pellicle candidates.
Meeting the durability specification at 157 nm remains a major concern. The 157 nm radiation durability lifetime of
a polymer is determined by two fundamental properties: the fraction of 157 nm radiation absorbed and the fraction (quantum
efficiency) of this absorbed radiation that results in photochemical darkening. Originally it was assumed that lifetime
increases uniformly with increasing transparency. We now have cases where materials with very different absorbances
(TAFx4P and 46P) have similar lifetimes and materials with similar absorptions (TAFx46P and 2P) have very different
lifetimes. These findings demonstrate the importance of the relative quantum efficiencies as the 157 nm light energy
distributes itself along degradative versus non-degradative pathways. In an effort to identify chemical and structural features
that control lifetime, we have been studying model molecular materials, some quite similar to the monomer units used to
make our pellicle candidates. Several of these models have shown transparencies much higher and lifetimes far longer than
our best pellicle candidates to date.

Transmission loss during irradiation remains the critical limitation for polymer pellicle materials at 157 nm. In this work we establish a framework for calculating the necessary pellicle lifetime as well as a test methodology for evaluating the laser durability of candidate polymer films. We examine the role of key extrinsic environmental variables in determining film lifetime. Oxygen concentration affects pellicle lifetime, but there is not an oxygen level that effectively balances pellicle perforation and cleaning against the onset of photochemical darkening. Neither moisture level nor 172-nm UV lamp pre-cleaning were found to have a significant impact on pellicle lifetime.

We present the results of a preliminary feasibility study of liquid immersion lithography at 157 nm. A key enabler
has been the identification of a class of commercially available liquids, perfluoropolyethers, with low 157 nm absorbance &alpha;<sub>157</sub>~10 cm-1 base10. With 157 nm index of refraction around 1.36, these liquids could enable lithography at NA~1.25 and thus resolution of 50 nm for k1=0.4. We have also performed preliminary studies on the optical, chemical, and physical suitability of these liquids for use in high throughput lithography. We also note that at longer wavelengths, there is a wider selection of transparent immersion liquids. At 193 nm, the most transparent liquid measured, deionized
water, has &alpha;<sub>193</sub> = 0.036 cm-1 base 10. Water immersion lithography at 193 nm would enable resolution of 60 nm with
k1=0.4.

Photodeposition rates for ten hydrocarbon species have been measured on CaF<SUB>2</SUB> substrates under 157-nm irradiation in the presence of ppm scale levels of oxygen. The species are representative of hydrocarbon based compounds observed in outgassing studies of common build materials used in 157-nm based lithographic systems. Photodeposition rates have also been measured for a subset of the hydrocarbon species on a MgF<SUB>2</SUB> thin film, six anti-reflective dielectric stacks, and fluorine doped fused silica for comparison with the results on CaF<SUB>2</SUB> substrates. Two contamination processes are observed. One is the formation of an equilibrium layer on the surfaces. The other is a quasi-permanent contamination which is most pronounced at elevated levels of contaminant.

A UV-lamp-based cleaning station, serving as a load-lock to a VUV spectrometer, has been used to evaluate the cleaning of hydrocarbon residues on 157-nm reticles. UV lamp based cleaning is found to be an effective tool to remove both nanometer scale layers of physisorbed and significantly more resilient highly conjugated 'graphitized' layers on the mask substrate. Slight changes in reflectance and surface roughness are observed on the chromium absorber indicating some degree of photo-oxidation is occurring during lamp cleaning.

Contamination rates of CaF<SUB>2</SUB> substrates in the presence of trace levels of toluene vapor and oxygen under 157-nm irradiation have been studied to determine conditions which prevent contamination films from depositing on optical elements in lithographic projection systems. A 2 - 3 monolayer thick deposit, causing a 1 - 2% transmission drop per surface, can readily form over a range of contaminant levels in the sub-ppm range and typical background oxygen levels. In addition, stable partial surface coverage can be supported with either lower concentrations of contaminant or conversely much higher levels of oxygen. Contamination rates are also higher at lower fluences, and thus contamination effects are expected to impact the projection optics more severely than beam delivery and illumination components. Finally, a permanent degradation in transmission of coated optics has been observed on anti-reflective coatings exposed to sub-ppm levels of toluene. Taken together, the results suggest that even with hydrocarbon based contaminants, where oxygen can be introduced into the beam-line in trace levels (i.e. hundreds of ppb) without significantly degrading transmission, toluene contaminant levels will have to be maintained in the ppb range or below.

In this work we present progress on the long-term evaluation of optical materials for 157-nm lithographic applications. We review the unique metrology capabilities that have been developed for accurately assessing optical properties of samples both online and offline, utilizing VUV spectrophotometry with in-situ lamp-based cleaning. We review the current status of bulk materials for lenses, such as CaF<SUB>2</SUB> and BaF<SUB>2</SUB>, and durability results of antireflectance coatings. Finally, we describe progress on materials testing of organic pellicles, both with 172-nm lamps as well as under 157-nm laser irradiation.

A 157nm interference lithography system which is capable of patterning features at sub-100-nm pitch has been implemented. Initial results demonstrate approximately 50 nm line and space patterns exposed in a commercial deep-UV photoresists. Little line edge roughness is observed, indicating that the intrinsic properties of the resist may meet CD-control requirements to at least 50 nm. In addition, this system may be used to measure the spatial coherence of the 157-nm F<SUB>2</SUB> laser source. Preliminary estimates show that the coherence length is approximately 40 micrometers .

Photolithography utilizing 157-nm excimer lasers is a leading candidate technology for the post-193-nm generation. A key element required for successful insertion of this technology is the near-term performance and long-term reliability of the components of the optical train, including transparent bulk materials for lenses, optical coatings, photomask substrates, and pellicles. For instance, after 100 billion pulses at an incident fluence of 0.5 mJ/cm<SUP>2</SUP>/pulse optical materials, of which the primary candidate is calcium fluoride, should have an absorption coefficient of less than 0.002 cm<SUP>-1</SUP>, and antireflective layers should enable transmission of 98.5 percent for a two-sided coated substrate. Modified fused silica has emerged as a viable option as a transparent photomask substrate, and several approaches are being explored for transmissive membranes to be used as pellicles.

In-situ laser cleaning is shown to be an effective tool for removal of organic contaminants on CaF<SUB>2</SUB> windows. To study laser cleaning in a controlled fashion, CaF<SUB>2</SUB> substrates were pre-contaminated with 5 to 10 nm of poly(methyl methacrylate), poly(4-hydroxy styrene), poly(norbornene), and poly((beta) -pinene) thin films. Irradiation of all the polymer films showed similar trends. Initially, a high rate of material removal occurs, which depends on the chemistry of the polymer. During this period, the material also undergoes significant bond rearrangement, forming a more tightly bound highly conjugated network. Removal of this residual 'graphitized' film is significantly more difficult, but can be accelerated by the presence of modest levels of oxygen. For oxygen concentrations between 10-1000 ppm, the measured removal rate is approximately 3 nm/(kJ/cm<SUP>2</SUP>) ppm oxygen. No effect on removal rate was observed as pulse energy or purge gas flow rate was varied over ranges expected to be used in practical systems.

Photolithography using 157-nm pulsed F<SUB>2</SUB> lasers has emerged as the leading candidate technology for the 0.1 micrometer lithography node for the post-193-nm generation. The extension of operating wavelength to the VUV range presents new challenges for thin film metrology tools, such as ellipsometers and spectrophotometers, most of which have not yet shown robust performance at high accuracy at wavelengths below 193 nm. Knowledge of material optical properties near 157 nm is essential for several areas of microlithography, such as (1) optimization of resist and bottom antireflectance coating (BARC) and lithographic performance modeling; (2) development of thin dielectric layers for lens coatings, including antireflectance, beamsplitter and high reflectance designs; and (3) development of resolution enhancement techniques, such as attenuating phase shifting masks. In this work we review our experience with VUV spectrophotometers, as well as techniques for obtaining stable reflection and transmission measurements necessary for deriving optical constants of thin films. In particular, we find that reliably accurate reflection data can be obtained only using absolute reflectance methods. Extraction of optical constants is performed utilizing global optimization methods with a commercially available software package. Kramers-Kronig- consistent dispersion relations are used to describe the material dielectric constants. We will present real and imaginary refractive index values of various thin films, as determined from reflection/transmission data into the deep UV wavelengths to as low as 140 nm. A separate study designed to understand scatter losses of materials at 157 nm will also be described. We have constructed a 157-nm laser-based scatterometer for obtaining bidirectional reflection distribution function (BRDF) measurements. By correlating scatter signals with total transmission losses, we are able to separate absorption from scatter effects.

We have completed a comprehensive evaluation of bulk materials designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 6 X 10<SUP>9</SUP>. The outcome of the study shows that most calcium fluoride materials should meet the industry lifetime targets for use in lens applications. Some fused silica material also appears to meet lifetime expectations of the industry; however, large grade-to-grade variability in both absorption and laser-induced densification has been observed. We also report on the impact of transient absorption in fused silica on lithographic dose control.

We update previously reported results on the absorption of optical materials and coatings for use in 157 nm based optical projection system. New results include the transmissions spectrum of a modified from of fused silica with suitable initial transmission for use as a mask substrate. We also report on a more systematic study of the effects of surface contaminants on optical components at 157 nm. We have modified our vacuum spectrometer to allow in- situ cleaning to enable a closer examination of purging requirements and cleaning procedures.

We have undertaken a systematic evaluation of both bulk material sand optical coatings designed for 193-nm lithographic applications. These studies are performed at realistic fluences and pulse counts in excess of 10<SUP>9</SUP>. Measurements of absorption is fused silica show a large variation in performance for different samples in both initial and laser-induced absorption. Calcium fluorides samples show less variation in laser-induced absorption and appear to be more stable under irradiation of 0.2-1 billion pulses. Laser-induced densification of fused silica appears to follow an empirical power law; however, an order of magnitude spread in densification is observed among grades. For optical antireflectance coatings, we have characterized the initial 'laser-cleaning' phenomenon for various coatings. We have observed that laser-cleaned coatings deposited on CaF<SUB>2</SUB> substrates exhibit higher initial optical losses at 193 nm than their counterparts on SiO<SUB>2</SUB> substrates. However, the losses for coatings on CaF<SUB>2</SUB> substrates are reduced over irradiation times of 0.2-1 billion pulses to final values comparable to their SiO<SUB>2</SUB> counterparts. Finally, we have characterized various catastrophic failures of coating material, such as induced losses, adhesion failure and laser-induced thinning.

We present an assessment of bulk fused silica and calcium fluoride, and of antireflective coatings for 193-nm lithographic applications. In the course of extensive marathon studies we have accumulated 1-5 billion laser pulses on over 100 bulk material samples at fluences from 0.2 to 4 mJ/cm<SUP>2</SUP>/pulse. The result show large variation in both initial and induced absorption of fused silica and in densification of fused silica. For antireflective coatings, there are samples that undergo no appreciable degradation when irradiated for &gt; 1 billion pulses at 15 mJ/cm<SUP>2</SUP>/pulse. However, initial losses in some coatings may be unacceptably high for lithographic applications.

We present an assessment of antireflective coatings for 193-nm lithography. Coatings from nine suppliers were exposed in a nitrogen ambient for up to 1.5 billion pulses at 15 mJ/cm<SUP>2</SUP>/pulse at 400 Hz. Sensitive metrology, developed for this study, included reflectance/transmittance measurements, in-situ ratiometric transmission measurements, and interferometric calorimetry for absorption measurements. The coatings from at least two suppliers withstood greater than 1 billion pulses with no observable degradation. Catastrophic damage observed on some samples included blistering and a dramatic transmission drop. Such damage occurred rather early (less than 100 million pulses).

We investigated laser-induced damage of pellicles for 193-nm lithography. We surveyed 193-nm-optimized material from three pellicle suppliers. Pellicles were irradiated under realistic reticle plane conditions (0.04 mJ/cm<SUP>2</SUP>/pulse - 0.12 mJ/cm<SUP>2</SUP>/pulse for up to 100 million pulses). Pellicles from two suppliers were found to meet lifetime requirements of the industry. Pellicles from the third supplier do not appear to meet the lifetime requirements. We present fluence scaling of pellicle damage and discuss effects of the ambient on pellicle degradation rates. We present results of the outgassing studies of pellicle material under irradiation using a separate gas chromatograph-mass spectrometer-based detection apparatus. From the results of these studies, we suggest possible photochemical pathways for pellicle degradation as a function of ambient.

Projection photolithography is widely expected to remain the main high-throughput patterning technology of microelectronic circuits in the next few years. As the critical dimensions of these devices shrink to 0.18 micrometers and below, the lithographic wavelength will decrease from 248 to 193 nm, and possible to 157 nm. This paper reviews the challenges posed by reducing the wavelength to below 200 nm, and the current state of the art in the critical areas of optical materials, photoresists, and high-resolution patterning.

The performance of argon fluoride excimer lasers is an important issue in determining the practical feasibility of 193-nm exposure systems. This paper presents a summary of the experience gained at MIT Lincoln Laboratory regarding the long-term performance of 193-nm lasers, used under conditions similar to those expected in production-type lithographic systems.

We investigate the effect of 193-nm radiation on commercially available pellicles for 248-nm lithography. Pellicles from two suppliers were irradiated at a realistic reticle plane fluence (0.1 mJ/cm<SUP>2</SUP>/pulse) for 50 million pulses. Analysis of transmission spectra revealed loss of pellicle material, decreased refractive index and increased absorption in various combinations depending on pellicle type and ambient. Although one of the two materials may be suitable for use at 193 nm, the other showed unacceptable degradation. We also quantified outgassing rates of organic species during irradiation, and observed greatly accelerated material loss in a pure nitrogen ambient compared with air. Yield rates of perfluorinated fragments and polymer product exhibited two-photon scaling behavior.

The trend in microelectronics toward printing features 0.25 micrometers and below has motivated the development of lithography at the 193-nm wavelength of argon fluoride excimer lasers. This technology is in its early stages, but a picture is emerging of its strengths and limitations. The change in wavelength from 248 to 193 nm will require parallel progress in projection systems, optical materials, and photoresist chemistries and processes. This paper reviews the current status of these various topics, as they have been engineered under a multi-year program at MIT Lincoln Laboratory.

We have optimized a positive-tone silylation process using polyvinylphenol resist and dimethylsilyldimethylamine as the silylating agent. Imaging quality and process latitude have been evaluated at 193 nm using a 0.5-NA SVGL prototype exposure system. A low- temperature dry etch process was developed that produces vertical resist profiles resulting in large exposure and defocus latitudes, linearity of gratings down to 0.175 micrometers , and resolution of 0.15-micrometers gratings and isolated lines.

The effect of 193-nm excimer laser radiation on pellicles designed for 248-nm use is studied. The pellicles are transparent at 193 nm as well. However, prolonged irradiation causes gradual thinning and eventual rupture of the pellicle. The rate of change depends on the fluence and on the total dose but is not affected by the presence of atmospheric oxygen. Two-photon absorption plays an important role in the interaction between the laser and the pellicle material. Extrapolation to the approximately 0.1 mJ cm<sup>2</sup>/pulse fluences expected to be used in 193-nm steppers indicates that these pellicles will change little for several years in a full-production environment.

he effect of 1 93-nm excimer laser radiation on pellicles designed
for 248-nm use is studied. The pellicles are transparent at 1 93 nm
as well. However, prolonged irradiation causes gradual thinning and
eventual rupture of the pellicle. The rate of change depends on the fluence and on the total dose but is not affected by the presence of atmospheric oxygen. Two-photon absorption plays an important role in the interaction between the laser and the pellicle material. Extrapolation to the approximately 0.1 mJ cm2lpulse fluences expected to be used in
1 93-nm steppers indicates that these pellicles will change little for 5everal
years in a full-production environment.

Excimer laser irradiation of fused silica is shown to induce gradual changes in the material, which affect its optical properties. These changes include visible fluorescence, formation of absorption bands at approximately 215 nm (E' centers), and increases in density and index of refraction. The magnitude of these effects varies initially as the square of the laser fluence and linearly with the number of pulses, indicating that they are the result of a two- photon absorption process. Pre-or post-irradiation treatments can be used to reduce the amount of laser induced degradation, especially the formation of color centers.

Synthetic UV-grade fused silica, crystalline fluorides, and dielectric coatings have been evaluated for transparency and durability at 193 nm. Most bulk materials eventually develop color centers, and fused silica also changes its density and index of refraction. However, the rate at which these changes occur and their magnitude vary strongly with material, grade, and other more subtle details. Careful selection and possibly pretesting are recommended, in order to ensure optimal matching between the intended application and the material properties.

The effect of 193-nm excimer laser radiation on pellicles designed for 248-nm use has been studied. The pellicles are transparent at 193 nm as well. However, prolonged irradiation causes gradual thinning and eventual rupture of the pellicle. The rate of change depends on the fluence and on the total dose, but is not affected by the presence of atmospheric oxygen. Two-photon absorption plays an important role in the interaction between the laser and the pellicle material. Extrapolation to the -0.1 mJ cm”2/pulse fluences expected to be used in 193-nm steppers indicates that these pellicles will change little for several years in a full production environment

The local modification of an integrated circuit (IC) requires in general the availability of three generic processes. First, a method for cutting conductors must be provided. Second, a process for depositing new conductors must be available. Finally, a means of opening via holes through the chip passivation to the underlying conductors is needed; this operation enables newly deposited conductors to make connections to the existing circuit elements, and also provides probe access to facilitate testing of the circuit.

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